1. Field of the Invention
The present invention relates to a method for determining the position of a magnetic element using linear Hall effect position sensors. The invention also relates to the device allowing this method to be implemented.
More particularly, the invention is applicable to the detection of the position of a gearbox selector of a gearbox of an automobile vehicle and allows the gear ratio which is selected by the driver to be determined.
2. Description of the Related Art
The selection of a gear ratio for an automobile vehicle is generally carried out, for a manual gearbox, by a gear stick situated in the passenger compartment of the vehicle which, by means of a transmission linkage connected to an axis for selection of the gearbox ratio, allows a gear ratio to be selected in the gearbox by various systems of gears, known to those skilled in the art and not detailed here. The latter is generally situated under the vehicle and connected to the wheels and to the engine.
The gear stick can move in two perpendicular directions. This example is limited to the case of a gearbox comprising six gear ratios: 1st, 2nd, 3rd, 4th, 5th gears and reverse gear R (cf. FIG. 2). “Speed of the vehicle” will refer to the gear ratio selected in the gearbox and “neutral” N the null gear ratio. The movement of the gear stick is transmitted, in the gearbox to the speed selection axis, also referred to as a gearbox selector, which itself moves in translation and in rotation relative to the longitudinal axis of the gearbox. By means of various mechanical gears, this translational and rotational movement of the gearbox selector allows the speed chosen by the driver to be engaged.
In order to perform certain functions of the vehicle, such as for example turn off the engine when the gearbox is in neutral in order to reduce polluting emissions, it is common to use a sensor for the position of the gearbox selector. This position sensor generally comprises a linear Hall effect cell, connected to an integrated electronic circuit. A linear Hall effect cell is understood to mean an element, made of conducting or semiconductor material, through which a current flows and which is subjected to a magnetic field perpendicular to this current, which produces an output voltage proportional to the magnetic field and to the current flowing through it. The measurement plane of the Hall effect cell is perpendicular to the magnetic field generated by the magnetic element. A linear Hall effect cell will be referred to as Hall effect cell in the following part of the description.
As illustrated in FIG. 1, the position sensor 60 is generally situated in the housing 10 of the gearbox 30, facing a magnetic element 50 situated on the gearbox selector 20 (by a support 40) which moves, when the speeds are selected, in rotation in the main direction Z, and in translation in the secondary direction X, with respect to the longitudinal axis A of the gearbox 30. The magnetic element 50 is oriented in such a manner that it creates a magnetic field  which varies with the direction of movement of the gearbox selector 20 that it is desired to detect. In FIG. 1, the magnetic field  is oriented perpendicularly to the longitudinal axis A, and transversely with respect to the gearbox selector 20, in order to measure the rotational movement in the main direction Z of the gearbox selector 20. The orientation of the magnetic field  is illustrated more clearly in FIG. 2 which shows a top view of FIG. 1. The magnetic field  y is oriented along the transverse axis B of the gearbox 30. A Hall effect cell 70 detects the variations in the magnetic field  induced by the translational movement or in rotation (as in the example illustrated in FIG. 1) of the magnetic element 50 and thus determines the position of the gearbox selector 20.
However, this detection can only be carried out in a direction of movement of the gearbox selector 20, the Hall effect cell 70 only being sensitive to a direction of the magnetic field  generated by the magnetic element 50. In order to detect whether the gear stick 80 is, for example, in neutral N, the Hall effect cell 70 and the magnetic element 50 are positioned for determining the rotational position, in the main direction Z, of the gearbox selector 20. As previously explained and illustrated in FIG. 2, the magnetic field  created by the magnetic element 50 and detected by the position sensor 60 is oriented along the transverse axis B of the gearbox 30. If the gearbox selector 20 rotates in the positive sense, in the main direction Z, this means that one of the even speeds (2nd or 4th gear) or reverse gear R is selected by the gear stick 80. If it rotates in the opposing sense, in other words negative, one of the odd speeds is selected (1st, 3rd or 5th gear), and finally, if it is centered on the longitudinal axis A, then the gear stick 80 is considered to be in neutral.
As illustrated in FIG. 2, in order to overcome the drifts in the magnetic element and also the variations in the distance between the magnetic element and the position sensor 60, it is common to use a ratiometric ratio between the output voltages of two Hall effect cells 70a and 70b, situated within a single position sensor 60. These Hall effect cells 70a and 70b are situated in the same measurement plane, in other words they are coplanar in the plane defined by the longitudinal axis A and the transverse axis B, and they measure the same magnetic field . Indeed, by using a ratiometric ratio between the output voltages of two Hall cells 70a, 70b, this effect is cancelled. This method is, for example, described in the document FR 2 926 881. In this document FR 2 926 881, the arctangent of the ratiometric ratio between the output voltages of two Hall cells is used in order to detect the position of the gearbox selector 20 in rotation, hence independently of certain variations.
At the present time, this detection along a single direction of movement is insufficient, and it is useful to be able to detect the speed selected precisely in order to improve the performance characteristics of the vehicle (noise, emissions) by anticipating for example the change in speed. It therefore becomes necessary to be able to detect not only the type of speed selected (even, odd, neutral), but also the actual speed, and hence to detect the movement of the gearbox selector 20 in the two directions of movement, in rotation and in translation, with respect to the longitudinal axis A of the gearbox.
For this purpose, it is usual to add (cf. FIG. 3) an additional magnetic element 500 on the gearbox selector 20, oriented in such a manner that it creates a magnetic field  in the second direction of movement, or secondary direction X, that it is desired to detect (in our example, the translation), together with a position sensor 600 situated facing the latter (for example on the housing 10 of the gearbox 30) comprising one or two Hall cells 700a and 700b. These are situated in the same measurement plane and oriented so as to measure this second magnetic field . These two Hall effect cells 700a and 700b thus each produce an output voltage proportional to the movement of the gearbox selector 20, in this case in translation, in the positive direction of the secondary direction X. This configuration (using a single Hall effect cell per sensor) is described in the document WO 97 46 815. By using two magnetic elements 50 and 500 and two position sensors 60 and 600 (each comprising one or two Hall effect cells), in other words by duplicating the device used to detect the movement of the gearbox selector 20 in one direction only, it is therefore possible to detect its movement in the two directions and hence to precisely determine the speed selected by the user via the gear stick. This solution is robust but costly, since it is necessary to add a magnetic element and a position sensor.